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Old 10th June 2007, 11:37 PM   #11
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Hi, Christer,

I will try to understand your post above. I don't get much from first reading .

Concerning PSRR, there a schematic that is not often used, but I think it is usefull. The VAS is differential Q66-Q67, and the most interesting part that it is supplied by a CCS (Q68).

http://www.diyaudio.com/forums/attac...amp=1180358772

Many uses differential VAS, but only put one resistor between emitors of Q66-Q67 to rail.

Only one resistor to power Q66-Q67 has a slight different operation than if we put CCS (Q68) for powering the VAS differential, because the common drop of R69-R70 will not change the VAS current (like if we use only one R in place of Q68 CCS).

This way, the VAS will only sense the differential voltages of R69-R70, and not amplifying the common voltage (depending on the quality of Q68-CCS)

Why this kind of CCT (differential VAS with CCS powered) seldom used?
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Old 11th June 2007, 04:59 AM   #12
CBS240 is offline CBS240  United States
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Quote:
Originally posted by Christer

snip .....Maybe the distorsion from Vc is not negligible, however, and maybe it has a more intrusive spectrum? Maybe this could be a starting point for understanding if and why CMRR is important?

Questions like this got me thinking about using seperate feedback loops for the CM signal(Vc) and the differential signal(Vd). It seems to me that a common mode signal error at the output would mis-balance the input differential by way of the NFB loop.
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Old 11th June 2007, 11:56 AM   #13
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Quote:
Originally posted by lumanauw

I will try to understand your post above. I don't get much from first reading .
I admit it is not the clearest thing I have written.

Would you prefer it in mathematical formulas rather than text?


Quote:
Originally posted by CBS240

Questions like this got me thinking about using seperate feedback loops for the CM signal(Vc) and the differential signal(Vd).
I cannot really see how it would be possible to distinguish the two components and even less how to feed them back separately. If there is actually a problem of the kind I speculated about, then the remedy is to have a high enough CMRR. My previous post was just an attempt to understand how a lowish CMRR might possibly impact on the sound. However, it is well known how to achieve high CMRR, so it shouldn't really be a problem. The exception might be that some people insist on using a resistor only instead of a CCS. On the other hand, some claim it sounds better that way, which is further food for thought: Does that perceived improvement have anything to do with lower CMRR?
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Old 12th June 2007, 04:20 AM   #14
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Hi Christer

It is odd sometimes what people think sounds good as it is really left up to opinion. Beauty is in the eyes of the beholder, some say. If only there was a perfect amplifier, perfect speakers, and a perfect listening room we all could use as a control for comparison. It seems certain music sounds better with certain distortions to certain people. IMO, very low distortion in general and lots of headroom is desirable for the classical genre.

In the differential VAS amp with a CCS bias differential input, any common mode signal, due to phase shift or whatever, should change the amount of current in both transistors of the differential VAS. IOW, the tail current of the VAS is dependent on common mode and tail current bias of the input stage. If the VAS tail current is biased with a CCS, then the voltage change on that CCS(emitters of VAS) should be dependent on the common mode current change & tail current of the input stage, leaving the circuit with no real stable reference for a DC Q point. Sensing this change in voltage with a J-fet gate, so as not to interfere with the bias current of the VAS, a feedback loop could be established by referring this signal back to the input CCS so it has reference to the common mode operation of both stages, and the VAS CCS which is a constant...
http://www.diyaudio.com/forums/attac...amp=1179682044 ...

My

On this subject I'm beginning to feel like I'm getting stranded on thin ice, and it's starting to crack. There are quite a few, more knowledgeable folks here than me and I haven't seen much that resembles this in a power amp topology.... CMRR may not be a major issue as reason it is not discussed much here. In reading some of your posts, it is obvious you are perspicacious in the subject of SS, your thoughts?
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Old 12th June 2007, 02:04 PM   #15
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Is that CMRR is partially responsible for some of the sonic quality terms (beautiful words for describing how a power amp sounds) like we used to read in reviews?
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Old 12th June 2007, 03:07 PM   #16
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This is a clever circuit. It is from patent #5,568,561. In left picture, if the +in and -in are not in the same magnitude (180deg phase difference), there will be difference in drop on R1 and R2.
Making the CCT like in the right picture, the drop on R1 and R2 is adjusted to be the same, no matter if +in and -in are not in perfect balanced magnitude.
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Old 12th June 2007, 03:18 PM   #17
Hartono is offline Hartono  Indonesia
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"Is that CMRR is partially responsible for some of the sonic quality terms (beautiful words for describing how a power amp sounds) like we used to read in reviews?"

yes beautiful words but I had my enough share of reviews, following what the reviewer says which gear sounds good, but turns out to be major dissapointment. Review is still useful .though ..sometimes.
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Old 12th June 2007, 03:23 PM   #18
Hartono is offline Hartono  Indonesia
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All this balanced thing looks pretty until one remember that the input properties of each op-amp must be matched up to very high frequency.
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Old 12th June 2007, 03:27 PM   #19
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Maybe A4 in the right picture doesn't need to be an opamp, a discrete follower with high input impedance and quite low output impedance will do the work.
But something is bothering me. It is feedback feeded to the front. Will it oscilate?
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Old 12th June 2007, 03:36 PM   #20
Hartono is offline Hartono  Indonesia
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using op-amp inside the feedback loop is ok as long as proper compensation/ caution is used. The circuit shown didn't show all the compensation details though.

"a discrete follower with high input impedance and quite low output impedance will do the work"

problem is matching the capacitance in the input up to high frequency.
Including stray capacitance, so differential amp is better used with matched transistor in one package, or differential (instrument) op-amp.

EDIT : By the way, op-amp in feedback loop increase distortion, compared to resistor.


Cheers,
Hartono
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